The present invention relates generally to MR imaging and, more particularly, to a method and apparatus to correct amplitude modulation in multi-echo acquisition. The present invention is particularly applicable with fast spin echo (FSE) imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
A number of imaging techniques have been developed to reduce scan time. Reduction in scan times has a number of advantages. For example, as scan time is reduced, patient throughput increases thereby allowing more subjects to be imaged in a given period of time. Additionally, it is generally well-known that some subjects, and in particular children, the elderly, and those that are claustrophobic, are prone to movement during the scanning technique. Despite repeated requests from the health care provider to the contrary, subjects often cannot resist the urge to move during the often lengthy scanning technique. This movement can introduce motion artifacts in the final reconstructed image thereby jeopardizing the diagnostic value of the final image. As such, scan time reduction has been shown to reduce subject motion induced artifacts.
One particular imaging technique that has been developed to reduce scan time is fast spin echo (FSE) imaging. FSE is a widely used technique because of its applicability for spin-spin weighted imaging, proton density imaging, and spin-lattice weighted imaging in relatively short periods of time. Moreover, FSE imaging may be implemented for neural imaging, body imaging, and extremity imaging.
FSE imaging utilizes a multi-echo, spin-echo pulse sequence where different parts of k-space are acquired by different spin echoes. For example, a four echo spin-echo sequence may be applied such that k-space is segmented into four sections. For example, the first echo may be used to fill a center of k-space, the second echo for k-space adjacent to the center, and so forth, with the data from the last spin-echo used to fill the outermost regions of k-space. Since four echoes rather than one are used to fill k-space, scan time, in this example, may be reduced four-fold.
Notwithstanding the advantages of FSE imaging, one particular drawback is ghosting that may occur in the final reconstructed image as a result amplitude modulation of the echo signal. Amplitude modulation may, for example, be caused by T2 decay along the multi-echo train. This ghosting is particularly problematic for high SNR imaging, such as with multi-channel array coils.
It would therefore be desirable to have a system and method capable of correcting for amplitude modulation in multi-echo acquisition.